Instrument landing system



reienied Jen. 1s, 1949 INSTRUMENT LANDING SYSTEM Donald S. Bond, Philadelphia, Pa., assigner to Radio Corporation of America, a corporation of Delaware Application November 30, 1945, Serial-No. 632.061

7 Claims. (Cl. 343-6) This invention relates to instrument landing obtained on the ground and transmitted to the 1 aircraft.

The principal object of the present invention is -i to provide improved methods of and means for determining continuouslythe position of an aircraft with respect to a predetermined glide path, transmitting the information to the aircraft,v and utilizing said transmitted information to provide indications on the craft, or automatic control thereof.

Another object is to provide a system of the described type wherein no complex equipment is required to be carried on the aircraft, the usual communication receiver being used to obtain position information from the ground station.

A further object is to provide animproved method of and means for transmitting position information from a ground station to an aircraft, and for converting thesignals received on the aircraft to a continuously presented and readily interpretable visual indication.

The invention will be described with reference to the accompanying drawings, in which:

Figure 1 is a schematic diagram of one embodiment of the instant invention, and

Figure 2 is an illustration of a typical visual may also be employed for automatic control of Athe craft.

Locator and tracking system Refer te' Figure 1. A highly directional antenna I is supported on trunnions or the like (not shown) for rotation in both azimuth and elevation, like a search light. The antenna I, in addition to being rotatable about vertical and horizontal axes, is arranged to scan continuously in a narrow conical pattern about its principal directive axis. This may be donevby means of a radiator il disposed eccentrically with respect to the focal axis of a paraboloidal reflector 5, on

a shaft 1, driven by a motor 9.

The antenna I is connected through a duplexing circuit or T/R box II to a pulse transmitter transmitter I3 operates, short-circuiting the input to the receiver I5.

The transmitter I3 is connected to an audio frequency oscillator I1, and provides a sharp pulse of radio frequency output in response to each cycle of the output of the oscillator I1.

The output circuit of the receiver I5 is connected to a gate amplifier I9. The amplifier I9 is normally biassed to cuto. but may be made operative by means of a further bias applied to it from a trigger Icircuit 2l. The trigger circuit 2| may be of the type described by Eccles and Jordan in Radio Review, volume 1, page 143 (1919) or a circuit of the quiescent multivibrator type.' It includes one input circuit connected to the receiver I5, and a second input circuit connected through a delay network 23 to the transmitter I3.

Each pulse from the transmitter I3 t'lows the trigger circuit 2i to one condition, hereinafter called on," after a short delay provided by the delay circuit 23, removing the cut off bias from the amplifier I9. Each pulse from the receiver I5 throws the trigger circuit 2l to its other stable condition, hereinafter called om again cutting oir the amplifier I9. The delay produced by the network 23 is at least sufncient to prevent the amplifier I9 from being operative during the transmission of a pulse, and may be adjusted to cause rejection of received' pulses reected from nearby objects. Thus the amplifier I9 acts as a "gate allowing only the desired re-V ceived pulses to pass through. A cathode ray oscilloscope 25 is also connected to the output of the receiver I5, for monitoring and adjustment of the system.

The output circuit of the gate amplier I9 is connected to a pair of balanced modulator circuits 21 and 29. A reference phase or alternator gg generator 3| is coupled to the antenna scanning spark gap or glow tube which ionizes when the shaft 1, providing one cycle for each rotation of the shaft. 'I'he alternator 3| is connected directly to the modulator 21, and through a 21, 1947, Patent No. 2,429,257, by Donald S. Bond and entitled Remote control systems, and also includes a filter or other means to accept A.C. components of the frequency of scanning and to attenuate other frequencies that may be present.

The motor 39 is provided with a shaft 4I for rotating the' antenna I in azimuth. The motor 39 is also coupled to the movable ta'p of a variable voltage divider 43. The output circuit of the balanced modulator 29 is connected through a switch 41 to a second servo amplier 49, similar to the amplier 31, which is connected to an ele-- vation servomotor The motor 5| is provided with a shaft 53 for rotating the antenna I in elevation, and is coupled to an adjustable voltage divider 55. i

The oscillator I1 is connected directly to one input of a trigger circuit 51, similar to the trigger circuit -2 I, and through va variable phase shifter 59 to the other input of the trigger circuit 51. The trigger circuit is turned on by the output of the oscillator-H substantially coincidentally with each pulse from the transmitter I3, and is turned off by the output of the phase shifter 5,9, after a period determined by the adjustment of the phase shifter.

The output of the trigger circuit 51 is applied through a resistor 6I to a resistor 63. The output of the trigger circuit 2| is applied through a resistor (i5 to the resistor 63, The connections of the trigger circuits 2| and 51 to the resistor 63 are such that their outputs oppose each other; l. e. when both trigger circuits are on, no voltage appears across the resistor 63, and when only one trigger is on, there is a voltage across the resistor 63, of polarity depending on which of the circuits 2| and 51 is on.

The resistor 63 is of low resistance compared to resistors 6I and 65, so .there is substantially no coupling between the trigger circuits 2| and 51 and therefore the trigger 51 cannot control the gate amplifier I9. The resistor 'S3-is connected through a switch 61 to the input of an amplifier 69, similar to amplifier 31. The output of the amplifier 69 is applied to a reversible range servomotor 1|. The motor'1I is provided with a shaft 13, coupled to the phase shifter 59 and to an adjustable Voltage divider 15.

The operation of the system thus far described is as follows: I

The oscillator I1 excites the transmitter I3 cyclically to provide a train of sharp pulses of high frequency energy, which are radiated by the antenna I. A portion of the transmitted energy gets through the T/R box I I to the receiver, producing a reference indication on the screen of the oscilloscope 25. Any aircraft which lie in the beam of the antenna I reflect back a portion of the transmitted energy, which also goes through the receiver I5, providing further indications whose relationship to the reference indication is a measure of the range.

Initially the switches 35, 41 and 61 are opened, and shafts 4| and 53 are rotated manually, or by manually controlled means, not shown, to train the. antenna I on the aircraft which is to be guided to a landing, The shaft 13 is also rotated, under manual control, to a position corresponding to the range of the selected craft. The Aswitches 35, 41 and 61 arethen closed.

The trigger circuit '2| is turned on shortly after each pulse is transmitted, and is turned olf in response to the reception of a pulse reflected from the aircraft, as described above. Thus the length of the period during which the trigger 2| is on is a' measure of the distance of the craft from the antenna I. The trigger 51 is similarly turned on with each cycle of the oscillator I1, and oi after a period depending onfthe adjustment of the delay between transmission and reception of each pulse becomes less, and the "on" periods of the trigger circuit 2| become .correspondingly shorter. This causes a series of voltage pulses across the resistor 63, each of a duration equal to the difference in the on periods of the trigger circuits 2| Iand 51, and o1' polarity correspondy ing to that of the output of the trigger circuit 51.

These voltage pulses are" amplified in the amplifier 69, and energize the motor 1I to drive the range shaft 13 to reduce the amount of phase shift provided by the phase shifter 59. This action continues until the on periods of the two trigger circuits are again equal. The shaft 13 is thus maintained substantially continuously at a position corresponding to the range of the aircraft.

As stated above, the beam of the antenna I scans in a conical pattern. As long as the aircraft remalns at the center of this pattern, the deviation of the beam from the craft will remain constant throughout the scanning cycle, and the amplitude of the received pulses will be independent of the instantaneous position of the beam. When the,center of the scanning pattern does not coincide with the line from the antenna to the aircraft, the amplitude of the received pulses varies cyclically with thel scanning, being greatest when the beam is directed toward the cra t.

Thus the received pulses are amplitude modulated at the rotation frequency of the shaft 1.

The phase of this modulation, with reference to the output of the alternator 3|. depends on the direction in which the aircraft is away from the center of the scanning pattern. If the aircraft y is to the right, for example, the pulse modulathe range shaft 13. Initially, the on periods of the two trigger circuits are equal and the resultant voltage across the resistor 63 is zero.

As the-aircraft cornes` closer to the antenna I,

tion will be in phase with the reference voltage from the generator 3|. The gate amplifier I9 rejects the transmitted pulses, accepting and amplifying only the received pulses.

The modulator 21 responds to the reference voltage and the received pulses to provide an output including a component of the scanning frequency whichis amplied in the amplifier 31 and applied to the motor 39. The output of the amplifier 31 is of such polarity as to energize the motor 39 to rotate the shaft 4| to turn the antenna I to the right. This recurs with each rotation of the scanning shaft 1 until the pattern is again centered on the aircraft. If the laircraft is to the left of the beam, the received pulses are modulated 180 out of phase with the reference voltage, lthe balanced modulator provides an output of opposite polarity, and the antenna is rotated to the left,

Similarly, if the craft is above or below the beam, the received pulses are modulated, but in quadrature phase with respect to the reference voltage. This provides no output from4 the modulator 21, but does produce output from the modulator 29, since itis in phase (or 180 out of phase) with the output of the phase shifter 33. The modulator 29 controls the motor 5I in the same manner as the modulator 21 controls the motor 39, adjusting the antenna inelevation to center the scanning pattern on the aircraft.

Both of the above-described operations may take place simultaneously, or separately as required, maintaining the conical scanning pattern centered substantially 4continuously on the craft. The shaft 4| is thereby automatically at a position corresponding to the azimuth of the aircraft, and the shaft 53 is similarly maintained at a position corresponding to the elevation of.

the craft with respect to the location of the antenna Data transmission system The voltage divider 15 isconnected across a any known device for providing a signal which varies in some characteristic, such as amplitude or phase or frequency, in response to variations in magnitude of the D.C. input from the volt- .y age dividers 15 and 19. One such devise is shown in Theory and Applications of Electron Tubes,

I by H. J. Reich, 1st edition published by McGraw- Y; Hill Book Company in 1939, in Figure1045, g' page 363. In this case the modulator 0I is'an audio frequency oscillator whose frequency may be varied between the limits of 500 and 1000 cycles per second, for example, by variations of the D.C. control voltage. Since the voltage beg tween the taps of the voltage dividers 15 and 19 is a function of the angular position of the range shaft 13. the frequency of the output of the modu-V lator is a'measure 4of the range.

The voltage divider 43 is connected like the voltage divider 15, to a D.C. source 83 and a voltage divider 85. The taps of the voltage dividers 43 and 85 are connected to a modulator 01, like the modulator 0|', but designed to operate over a different band of audio frequencies, for example 1500-2000 cycles per second. The voltage divider 85 is variable, with its movable tap mechanically coupled to a cam 89 on the range shaft 13. The frequency of the output of vthe modulator 01 is thus a function both of the range and the azimuth.

The voltage divider 55 is connected, like the voltage divider 43, to-a D.C. source 9| and a voltage divider 93. The voltage divider 93 is controlled by a cam 95 on the range shaft 13, and the taps of voltage dividers 55 and 93 are connected-to a modulator 91. The modulator 31 is similar to modulators 8| and 01, but operates over a different band of frequencies, such as 2500 to 3000 cycles per second. The frequency of the output of the modulator 91 is a function both of the elevation and the range.

The output circuits of the modulators 8|, B1 and 91 are connected through a switch 99 to a radio transmitter |0|. The transmitter |0| is of the type ordinarily used for voice communication between aircraft and the ground station, and may be connected to a microphone |03 for this purpose by means of the switch 99. V

A receiver |05 is provided on board the aircraft. The receiver |05 is of the usual voice communication type and is provided with transducer means such as headphones |'01. Three band pass filters |09, III', and ||3 are connected to the output circuit of the receiver |05, and are designed to pass the bands corresponding to the outputs of modulators 8|, 81 and 91 respectively.

The filter |09 is connected to a demodulator H5, designed to provide a D.C. output corresponding to the D.C. input to the modulator 8|'. In the present example, where the modulator 8| is a variable frequency device, the demodulator i5 is a frequency responsive circuit, such as a discriminator or cycle counter.

Demodulators ||1 and ||9,simiiar to the demodulator 5, are connected to the filters iii' and ||3 respectively. These provide D.C. out- .nected to a double throwv switch |25.

puts corresponding to the inputs to the monulators 81 and 91 respectively.

The operation of the described data transmission system is as follows:

The modulator 8| provides output of frequency corresponding to range, as described above. This signal is transmitted by way of the communication equipment to the aircraft, where it is demodulated to provide a signal corresponding to the range. The azimuth and elevation channels operate the same way, except for the effects of the cams 39 and 95.

Owing to the facts that the preferred glide path is not necessarily a straight line, and that the antenna cannot be located at the desired point of contact of the aircraft with the surface, the angle of elevation of the glide path is a function of range. 'I he cam 95 is designed in accordance with the required variation of elevation with range, so that the input to the modulator 91 is always the same regardless of range, when the aircraft is on the correct glide path.

Similarly, it may be necessary to locate the antenna to one side of the glide path, rather than in line with the runway. This causes the azimuth to vary with range, and the variation is compensated by the cam 09. It will be apparent that if the antenna is in line with the runway, the cam 09 could be omitted and the voltage divider could be fixed. Thus the inputs to the modulators 81 and 91, and hence the outputs-of the demodulators ||1 and H9, correspond to the azimuth and elevation of the aircraft with respect to the desired glide path, rather than with respect to a. straight line through the antenna.

Converter and indicator system A cathode-ray oscilloscope |2| is provided on the aircraft, with conventional power supply means, focussing, intensity and beam centering adjustments, not shown. The vertical deflection circuit is connected to a double throw switch |23, and the horizontal deflection circuit is con- The switches |23 and |25 are mechanically ganged for cyclical operation by a cam |21 driven by a motor |29.

The demodulators ||9 and ||1 are connected to the upper xed contacts of switches |23 and |25 respectively. The lower contact of the switch |23 is connected to an altimeter |3I, preferably of the frequency modulation type, which provides a D.C. output proportional in magnitude to the altitude of the aircraft.

The lower contact of the switch |25 is connected to the output of a modulator |33. 'Ihe modulator |33 is connected to an oscillator |35, whose frequency is several times the frequency of operation of the switches |23 and |25. The output of the demodulator ||5 is also applied to the modulator |33, in such polarity as to reduce the output of the modulator |33 in response to increase in output of the demodulator H5.

The operation of the convertor and indicator system is as follows:

When the switches |23 and |25 are in the upper position, the position of the cathode ray in the tube |2| is determined by the outputs of the azimuth and elevation channels. As long as the aircraft is on the glide path, the resultant spot on the screen of the cathode ray tube is at the center. VIf the craft is to the right of the glide path, the spot is deected to the left. If the craft is above the glide path. the spot is deiiected down. Refer to Figure 2. A fixed target I 31 tion in azimuth from the glide path. The vertical distance B similarly indicates the deviation in elevation.

When the switches |23 and |25 are in the lower position, the cathode ray beam is swept rapidly from side to side by the output of the modulator |33. This produces a. luminous line |4| (Figure 2) on the screen. The output of the altimeter depresses the beam in accordance with the altitude, so that the distance H of the line |4| below the target I 31 is a measure of the altitude. Since the output of the modulator |313 is controlled inversely by that of the range channel, the length L of the line |4v| increases as the range decreases.

The switches |23 and |25 operate at a high enough rate to make both of the above-described indications appear to be present simultaneously. The pilot controls the aircraft to keep the spot centered on the target. The line MI presents the appearance of a lineon the ground, transverse to the runway, giving the illusion of perspective by increasing in length as the range decreases. As the glide path is followed to the ground, the line IM rises with decrease in altitude, reaching the target |31 when the craft-arrives at the point of contact. Thus a readily tively to the range of saidcraft from said ground station and to the deviation in azimuth and elevation of said craft with respect to said glide path, cathode ray oscilliscope means, means for applying intermittently said azimuth and ele- T vation signals simultaneously to said oscilloscope to provide a visual indication of the position of said craft with respect to said glide path, and means for applying intermittently said range signal to said oscilloscope to provide visual indication of Saidrange.

'2. An instrument landing system for aircraft including a ground station, a radio locator system at said ground station including means for determining substantially continuously the azimuth, elevation, and range of an aircraft, means responsive to said range information to modify said azimuth and said elevation information to pro- V vide modulation signals corresponding .respecinterpretable Visual indication of the position of L the aircraft is provided continuously on the cathode ray tube I2I. The demodulators and ||9 may be connected to an automatic pilot mechanisrn, not shown, to guide the craft automatically to a landing.

The invention has been described as an instrument landing system for aircraft, wherein a; radio locator system is provided at a ground station, with means for tracking an approaching aircraft. The elevation and azimuth of the craft are measured continuously, and their relationships to the desired glide path are computed and transmitted by radio to the aircraft. A convertor system on the aircraft provides deflection voltages for a cathode ray tube in response to the signals from the ground station, to produce a simple visual display of the position of the craft with respect to the glide path and the point of contact.

I claim as my invention: l

l. An instrument landingsystem for aircraft including a ground station, a radio locator system at said ground station including means for determining substantially continuously the azimuth, elevation, and range of an aircraft, means responsive to said range 'information to modify said azimuth and said elevation information to provide modulation signals corresponding respectively to the deviation in azimuth and in elevation of said craft.from a predetermined glide path, means responsive to said range information to provide a modulation signal correspondingl to the range of said craft, a radio transmitter,

and means for applying all of said modulation signals to said transmitter; a mobile station on said aircraft including a radio receiver responsive to said ground station transmitter to provide an output similar to the combined modulation inout to said transmitter, means for separating and separately detecting the components of said output to provide signals corresponding respectively to the dev-lation in azimuth and in elevation of said craft from a predetermined glide path, means responsive' to said range information to provide a-modulation signal corresponding to the range of said craft, a radio transmitter, and means for applying all of said modulation signals to'said' transmitter; a mobile station on said aircraft including a radio receiver responsive to said ground station transmitter to provide an output similar to the combined modulation input to said transmitter, means for separating and separately detecting the components of said output to provide signals corresponding respectively to the range of said craft from said ground station and to the deviation in azimuth and elevation of said craft with respect to said glide path, cathode ray oscilloscope means, means .for applying intermittently'said azimuth and elevation signals simultaneously tol said oscilloscope to provide a visual indication of the position of4 said craft with respect to said glide path, an altimeter including means providing a further signal corresponding to the altitude of said craft, and means for applying intermittently said range signal and altitude signallsimultaneously to said oscilloscope tov provide visual indication of said altitude and range.

3. An instrument landing system for aircraft including a ground station. a radio locator sysup system responsive to said receiver to maintain' said antenna aligned in elevation upon said craft, a range shaft, further follow-up means responsive to the relative timingof signals as transmitted by said transmitter and as received Iby said receiver to maintain said range shaft in a position corresponding to the distance of said craft from said ground station, means responsive to said range shaft and to said azimuth follow-up system to provideA a modulation signal corresponding to the deviation inlazimuth of said craft from a predetermined glide path. means responsive to said range shaft and to saidelevation follow-up system to provide a second modulation signal corresponding to the deviationl in elevation' of said craft from said glide path. means responsive to said range shaft to provide a third modulation .signal corresponding to the distance of said craft from said ground statiom vation of said craft with respect to said glide path, cathode ray oscilloscope means, means for applying intermittently said azimuth and elevation signals simultaneously to said oscilloscope to provide a visual indication of the position of said craft with respect to said glide path, and means for applying intermittently said range signal to said oscilloscope to providevisual indication of said range.

4. An instrument landing system for aircraft including a ground station, a radio locator system at said ground station, said radio locator including a transmitter, a receiver, and a directive antenna, a follow-up system responsive to said receiver to maintain said antenna aligned in azimuth upon a selected aircraft, a second followvup system responsive to said receiver to maintain said antenna aligned in elevation upon said craft, a range shaft, further follow-up means responsive to the relative timing or signals as `transmitted by said transmitter and as received by said receiver to maintain said range shaft in a position corresponding to the distance of said craft from said ground station, means responsive to said range shaft and to said azimuth follow-up system to provide a modulation signal corresponding to the deviation in azimuth of said craft from a predetermined glide path, means responsive to said range shaft and to said elevation follow-up system to provide a second modulation signal corresponding to the deviation in elevation of said craft from said glide path, means responsive to said range shaft to provide a third modulation signal corresponding to the distance of said craft from said ground station,

system for deriving substantially continuously information as to the range of said craft, means responsive to said range information to modify said azimuth infomation and said elevation information in accordance with the characteristics of a, predetermined glide path, means for converting said modified azimuth and eleivation information and said range information into individually characteristic signals, means foi` transmittingsaid signals to said aircraft, and means on said aircraft responsive to said signals for providing indication of the deviation in flight of said craft from said glide path.

6. A system for guiding aircraft, including, at a ground station, a radio locator system providing substantially continuously information as to the direction and range of an aircraft from said ground station, means responsive to said range information to modify said direction .information in accordance with the characteristics of a predetermined flight path, means for converting said modified direction information into characteristic signals, and means for transmitting said signals to said aircraft.

7. A system for guiding aircraft, including a radio locator system including directive radio transmitter and receiver means, follow-up systems responsive to the output 'of said receiver means to maintain the directive axis of said radio a radio communications transmitter, and means for applying all of said modulation signals to said communications transmitter; a mobile station on 'said aircraft including a radio receiver responsive to said ground station transmitter to provide an output similar to the combined modulation input to said transmitter, means for separating and separately detecting the components of said out- .put to provide signals corresponding respectively to the range of said craft from said ground station and to the deviation in azimuth and elevation of said craft with respect to said glide path. cathode ray oscilloscope means, means for applying intermittently said azimuth and elevation signals simultaneously to said oscilloscope to pro- V vide a visual indication of the position of said craft with respect to said glide path, altimeter means providing a further signal corresponding to the altitude of said craft, and means for applying intermittently said range signal and said altitude signal to said oscilloscope to provide visual indication of said range.

5. An instrument landing system for aircraft including, at a ground station, a radio locator system including directive radio transmitter and receiver means, means for automatically maintaining the directive axis of said radio locator system in substantial alignment with a selected aircraft, means responsive to said last-mentioned means to derive substantially continuously infomation as to the azimuth and the elevation of said craft, means responsive to said locator locator system in substantial alignment with a selected aircraft, two voltage sources and voltage control devices therefor responsive to said respective follow-up systems to provide respective voltages substantially proportional to the deviation vertically and horizontally respectively of an aircraft from a. predetermined glide path, and a further follow-up system vresponsive to the time delay between transmission of a signal from said radio locator system and reception of said signal by said radio locator system after reection by said aircraft to provides. voltage approximately inversely proportional to the range of said craft from a predetermined point on said path; a cathode ray oscilloscope including a fluorescent screen, a stationary target approximately at the center of said screen, and cathode ray deflection to said target corresponding to the positionalrelationship between said glide path and said craft, and to apply to said last-mentioned voltage to said ,deflection elements to periodically deilect said cathode ray to produce a luminous line on said screen corresponding inversely in length to the distance of said craft from said point. DONALD S. BO'ND.

REFERENCES crrap The following references are of record in the le of this patent:

UNITED STATES PATENTS 

